Digital pre-distortion (DPD) is a technique used to linearize a power amplifier in a transmitter to improve the efficiency of the power amplifier. A digital pre-distortion circuit inversely models the gain and phase characteristics of the power amplifier and, when combined with the amplifier, produces an overall system that is more linear and reduces distortion than would otherwise be caused by the power amplifier. An inverse distortion is introduced into the input of the amplifier, thereby reducing any non-linearity that the amplifier might otherwise exhibit.
PCT Patent Application No. PCT/US12/62186, filed Oct. 26, 2012, entitled “Processor Having Instruction Set with User-Defined Non-Linear Functions for Digital Pre-Distortion (DPD) and Other Non-Linear Applications,” discloses non-linear functions that include one or more parameters specified by a user, such as filter coefficient values or values from a look-up table. The disclosed DPD techniques are based on a generalized memory polynomial (GMP) model and implemented using look-up tables. Polynomial models, however, do not provide adequate performance for a number of common situations. For example, polynomial models do not capture functions that have discontinuities in them (such as a discontinuity of amplitude or derivative, or a higher order derivative).
Chao Lu et al., “A 24.7 dBm All-Digital RF Transmitter for Multimode Broadband Applications in 40 nm CMOS,” IEEE Int'l Solid-State Circuits Conference (ISSCC) 19.3 (San Francisco, 2013) describes a technique for using two-dimensional look-up tables to implement a non-linear function with a complex input variable, selecting a value of a complex valued non-linear function based on the input value comprised of real and imaginary components. Generally, two-dimensional look-up tables avoid the need to compute the magnitude of the complex signal and also allows the potential I/Q mismatch in the RF signal to be taken into account in Zero IF (ZIF) architectures. Two-dimensional look-up tables, however, have a complexity on the order of N2, where N is the number of discrete points for each of the real or imaginary parts. Thus, the cost of the silicon and power concerns are issues to be addressed, particularly in battery operated handset applications.
A need therefore remains for improved techniques for modeling non-linear systems using two-dimensional look-up tables with reduced size.